Microsoft has extended the availability of Windows XP on new PCs by six months, the company confirmed Friday.

Computer makers that "downgrade" machines from Windows Vista Business or Vista Ultimate to Windows XP Professional will be able to obtain media for the latter through the end of July 2009, a Microsoft spokeswoman said Friday.

The new date is a change in policy. Previously, Microsoft had planned to halt XP Professional media shipments to major computer makers after Jan. 31. 2009.

"As more customers make the move to Windows Vista, we want to make sure that they are making that transition with confidence and that it is as smooth as possible. Providing downgrade media for a few more months is part of that commitment," the spokeswoman said in an e-mail.

The Jan. 31, 2009 date is also the last day when smaller companies, dubbed "system builders," will be allowed to purchase Windows XP licenses to install on the machines they assemble. The system builder deadline has not changed, the spokeswoman added; It remains next Jan. 31.

To confuse matters, some PC makers have long claimed that they would provide XP downgrades on new computers past the Jan. 31 deadline. Last June, for example, Hewlett-Packard talked of a July 2009 cut-off. "HP...will continue to offer this option on its business systems through at least July 30, 2009," a company spokesman said almost four months ago.

The Microsoft spokeswoman clarified the situation. "The [downgrade] rights don't go away," she said via instant messaging in response to follow-up questions. "It's all about having the media on hand. It's always been okay to use what you've got."

Microsoft sent Windows XP into semi-retirement last June when it stopped selling the aged operating system at retail, withdrew Windows XP Home from use on new PCs and allowed XP Professional to be installed as a Vista downgrade.

The latter tactic takes advantage of Vista's end-user licensing agreement (EULA), which allows users -- and in their stead, computer makers -- to install Windows XP Professional while also providing media for Vista for a possible upgrade later. More than a third of all new PCs are being downgraded to Windows XP, according to data from a Florida company that operates a community-based performance testing network.

It's also possible that XP will be widely available long after July 31, 2009. "Downgrade rights do not expire," Microsoft's spokeswoman said Friday.

The longer availability puts Microsoft in an unusual position; the new timeline will make it possible for users to purchase XP-powered PCs through next July, just months before Microsoft plans to roll out Windows 7, the successor to Vista.